About NPC head parts

Head parts work fine for NPCs without valid facegen data, but this isn't very useful because it triggers the infamous dark face bug. Special restrictions apply to NPCs that do have facegen data:

• Only one XML file can be used per face, even if was built from multiple physics-enabled parts.
• NiStringExtraData nodes aren't automatically copied into the facegen file, so physics won't work automatically. Either do it manually in NifSkope or map one of the head parts to a file in defaultBBPs.xml.
• Bones that aren't explicitly referenced by any mesh are removed when facegen data is generated, and can't be used as kinematic objects in constraints. Replace references to these with the NPC head (which should always be present). You may also need to set the frame of reference origin to the position of the missing bone relative to the head to get correct constraint behavior.